1. Field of the Invention
The invention relates to a heat-insulating protective layer and, more particularly, to a heat-insulating protective layer for a component within the hot-gas section of a gas turbine.
2. Description of the Related Art
In modern gas turbines, almost all of the surfaces in the hot-gas section of the turbine are provided with coatings. Exceptions to this may still be found in the turbine blades in the rear of a turbine blade array. The heat-insulating layers serve to lower the material temperature of the cooled components. As a result, the service life of the components can be extended, cooling air can be reduced, or the gas turbine can be operated at higher inlet temperatures. Heat-insulating layer systems in gas turbines always consist of a metallic bonding layer which is diffusion bonded to the base material, on top of which a ceramic layer with poor thermal conductivity is applied, which provides the actual barrier against the heat flow and protects the base metal of the component against high-temperature corrosion and high-temperature erosion.
Zirconium oxide (ZrO2, zirconia) has become widely accepted as the ceramic material for the heat-insulating layer, which is almost always partially stabilized with approximately 7 wt.% of yttrium oxide (international abbreviation: “YPSZ” for “Yttria Partially Stabilized Zirconia”). Here, the heat-insulating layers are divided into two basic classes, depending on how they are applied. The first class comprises thermally sprayed layers (usually applied by the atmospheric plasma spray (APS) process), in which, depending on the desired layer thickness and stress distribution, a porosity of approximately 10-25 vol.% in the ceramic layer is produced. Binding to the (raw sprayed) bonding layer is accomplished by mechanical interlocking. The second class comprises layers which are deposited by the EB-PVD (Electron Beam Plasma Vapor Diffusion) process, which, when certain deposition conditions are observed, have a columnar or a columnar elongation-tolerant structure. Here, the layer is bound chemically by the formation of an Al/Zr-mixed oxide on a layer of pure aluminum oxide, which is formed by the bonding layer during the application process and then during actual operation (Thermally Grown Oxide, TGO). This imposes very strict requirements on the growth of the oxide on the bonding layer.
In principle, either diffusion layers or cladding layers can be used as bonding layers.
The list of requirements on the bonding layers is complex and includes the following conditions which must be taken into account: i) low static and cyclic oxidation rates; ii) formation of the purest possible aluminum oxide layer as TGO (in the case of EB-PVD); iii) sufficient resistance to high-temperature corrosion; iv) low ductile-brittle transition temperature; v) high creep resistance; vi) physical properties similar to those of the base material, good chemical compatibility; vii) good adhesion; viii) minimal long-term interdiffusion with the base material; and ix) low cost of deposition in reproducible quality.
For the special requirements in stationary gas turbines, bonding or cladding layers based on MCrAlY (M=Ni, Co) offer the best possibilities for fulfilling the chemical and mechanical conditions. MCrAlY layers contain the intermetallic β-phase NiCoAl as an aluminum reserve in a NiCoCr (“γ”) matrix. The β-phase NiCoAl, however, also has an embrittling effect, so that the Al content which can be realized in practice is ≦12 wt. %. To achieve a further increase in the oxidation resistance, it is possible to coat the MCrAlY layers with an Al diffusion layer. Because of the danger of embrittlement, this is limited in most cases to starting layers with a relatively low aluminum content (Al≦8%).
The structure of an alitized MCrAlY layer consists of the inner, extensively intact γ, β-mixed phase, a diffusion zone, in which the Al content rises to ˜20%, and an outer layer with a β-NiAl phase, with an Al content of about 30%. This outer layer with a NiAl phase represents the weak point of the layer system with respect to brittleness and crack sensitivity.
In addition to the oxidation properties and the mechanical properties, the (inter)diffusion phenomena between the base material and the MCrAlY layer— in specific cases also between the MCrAlY layer and the alitized Layer— become increasingly more important with respect to service life as the service temperature increases. In extreme cases, the diffusion-based loss of aluminum in the MCrAlY layer can exceed the loss caused by oxide formation. Through asymmetric diffusion, in which the local losses are greater than the supply of fresh material, defects and pores can form and, in the extreme case, the layer can delaminate.